The popping sound could be bad bottom bracket bearings, or a bad freewheel. You need to be a genius mechanic to get inside a freewheel to check it - so start with a check on the BB bearings. The races may be shot. If everything's fine - you might want to just buy a new freewheel. Also - check and make sure your front and back cogs are properly aligned. If one of the cogs is crooked, exerting pressure may cause the chain to jump on the cogs enough to make a noise - but not enough to knock the chain off.